Modern transformer windings are fabricated using a wide variety of methods. In high power applications, a rectangular shaped ribbon conductor may be wound about a core to form a coil. Adjacent coil sections may be coupled by complicated transposition folds of the ribbon conductor.
A typical ribbon conductor comprises parallel strands of insulated wire resulting in a wide, thin cable. The strands themselves may be rectangular for increased strength, and to provide a more compact transformer. For instance, the ribbon conductor may comprise five strands or ribbon conductor elements. The strands may be approximately 0.045 inches by 0.250 inches, with a typical range of 0.030 to 0.096 inches by 0.200 to 0.580 inches. The strands are coated with enamel. The ribbon conductor is then wrapped in an insulating paper.
When ribbon conductors are used in core-form transformers, the thinness of the conductors allows more turns to be wound into a coil section, resulting in a winding that has a fraction of the sections of a conventional coil. The greater width of the ribbon conductor allows a high series capacitance in the coil. This provides an improved voltage distribution across the coil sections and permits reduced section and turn insulation. The reduced insulation and the fewer coil sections result in a greatly improved space factor for the coil and hence a smaller core with lower winding weights and lower no load and load losses.
While ribbon conductors may be successfully employed to manufacture core-form transformers, there are still a number of shortcomings associated with existing devices. One shortcoming is that they are susceptible to mechanical weakness. In addition, it would be highly advantageous to further reduce the size of core-form transformers without reducing their capacity.
When core-form transformers are manufactured from ribbon conductors, a number of inefficiencies are associated with the manufacturing process. First, the insulating paper wrapped around the conductor typically forms a bulging and wasteful overlapping paper region on one side of the conductor. In addition, the creation of transpositions between coil sections typically entails a labor-intensive folding scheme.
In addition to the structural and manufacturing shortcomings of existing core-form transformers, there are some operating inefficiencies associated with these devices. Several factors influence transformer efficiency, but transformer efficiency is largely dependent upon reducing eddy currents and circulating currents within the windings. It is recognized that eddy currents are dependent to a large extent on the dimensions of the conductors. Specifically, eddy current losses may be significantly reduced by reducing the dimensions of the conducting strands. Ribbon conductors comprised of a number of finely stranded conductors, as previously described, significantly reduce eddy currents.
However, to prevent an offsetting increase in losses due to circulating currents between the parallel connected strands, the leakage flux must be minimized. This is accomplished by transposing the relative position of the strands in order to reduce the net flux linkages for each strand. In conventional transformers, circulating currents are reduced by placing a transposition in between essentially each coil section of the transformer. This approach is problematic in that it involves a large number of transpositions which, as previously discussed, are difficult and expensive to realize. The approach is also problematic to the extent that it does not achieve optimal reduction of circulating losses.